Why did the Matthew Stafford-Jared Goff trade become official today, not yesterday?

Goff had a $2.5 million roster bonus due on the second day of the league year, which is today. The #Rams agreed to pay it as part of the trade to the #Lions, per source. A parting gift from L.A.

— Tom Pelissero (@TomPelissero) March 18, 2021